anhedral

Sometimes described as 'negative dihedral', anhedral refers to an aircraftwing configuration where the wing tips are lower than the root, or center, where they meet each other or the fuselage. Most balsa wood toy airplanes, both gliders and rubber band powered types, have dihedral wings. The AV-8B Harrier 'jump jet' is an example of a modern plane with anhedral wings. Dihedral contributes to the inherentstability of an aircraft, while anhedral reduces inherent stability.

The liftvector of a wing is generally perpendicular to the plane of the wing. When an aircraft with dihedral wings is banked to one side, the lower wing's lift vector is more perpendicular to gravity than it was before, and much more than the other wing, so the lower wing tends to lift more - this restoring forcerolls the plane back to level flight (without further control input). An aircraft with anhedral will conversely have the higher wing producing the most force perpendicular to gravity, and will thus tend to settle to a certain bank angle to one side or the other (and almost certainly will thus be in a turn as well).

Hang gliders employ very small amounts of anhedral in order to be able to be controlled in the roll axis by the relatively small forces of pilot weight shift. Wing sweep and both physical twist and aerodynamic twist make hang gliders stable in yaw and pitch, respectively, and also contribute to roll stability a bit, which is countered by anhedral. Control surfaces, such as ailerons, are aerodynamic controls, and thus easily overcome the strong restorative force of dihedral. Hang gliders by definition lack aerodynamic controls and so cannot use dihedral. By adjusting the mix of sweep, twist, and anhedral, a hang glider can be designed to be very stable, rolling back to level flight firmly with more roll resistance at higher angles, or it can be designed to prefer a given bank angle, or can even roll to level from shallow bank angles and have very light roll pressure at higher bank angles. Since hang gliders spend a lot of time circling in lift (like hawks and eagles, which also exhibit anhedral (vultures have dihedral)) such fine-tuned combinations are preferred.